Exemplary embodiments of the present invention relate to a method for operating a motor vehicle internal combustion engine.
German patent publication DE 102009021114.4 describes a method for operating an internal combustion engine with an exhaust tract featuring an exhaust gas purification unit which acts catalytically and/or by filtration in which fuel and a combustion gas featuring a part containing air and a part containing exhaust gas that is recirculated from the exhaust tract are fed to the combustion chambers of the internal combustion engine. At the same time, the recirculated exhaust gas can be supplied to the internal combustion engine with a low-pressure proportion via a low-pressure path diverging from the exhaust tract downstream of the exhaust gas purification unit, and to the combustion chambers with a high-pressure proportion via a high pressure path diverging from the exhaust tract upstream of an exhaust gas turbocharger turbine. By appropriate adjustment of the low-pressure proportion and the high-pressure proportion of the recirculated exhaust gas, the method disclosed in DE 102009021114.4 also enables, among other things, an effective purification of the exhaust gas, wherein, however, the effectiveness of changes that occur over the operating period of the exhaust gas purification unit are not taken into account.
By contrast, the exemplary embodiments of the present invention provide a method for operating a motor vehicle internal combustion engine that enables the purification of exhaust gas that is as constant as possible over the operating period of the exhaust gas purification unit.
The inventors have recognized that when calibrating a low-pressure proportion of recirculated exhaust gas that essentially decreases with an increased operating period, in the case of operating points of the internal combustion engine that are at least almost the same, efficiency changes occur over time in the exhaust gas purification unit, which can be compensated in an advantageous manner. This results in improved stability of the purification performance. In particular, it is possible to compensate an ageing-related decrease in the catalytic efficiency of the exhaust gas purification unit. One explanation for this surprising effect is at least partially to be seen in an ageing-compensating reduction in the flow of exhaust gas through the exhaust gas purification unit and in a favorable influence on the thermal conditions in the exhaust gas purification unit.
The method can be applied especially advantageously in an air compressing internal combustion engine, but also in the case of internal combustion engines with spark ignition. The exhaust gas purification unit, which acts catalytically and/or by filtration, preferably comprises an oxidation catalytic converter and a particle filter.
The low-pressure proportion of recirculated exhaust gas that essentially decreases with an increased operating period means a process that is comparatively slow to change and which typically extends over a total operating period of the exhaust gas purification unit provided. In connection with this, the entire operating period is assumed to represent a distance of 100,000 km to 200,000 km (for the corresponding vehicle). Preferably, the decrease in the low-pressure proportion is linked to a decrease in the purification performance and/or the catalytic efficiency of the exhaust gas purification unit that usually occurs over the operating period as a result of ageing. The decrease in the in the low-pressure proportion of the recirculated exhaust gas takes place at least in the main part of the operational range of the internal combustion engine, preferably continuously over the operating period of the exhaust gas purification unit. At least a decreasing trend for the low-pressure proportion of the recirculated exhaust gas over the operating period is provided. At the same time, a greater decrease can take place during certain operating periods than during other operating periods. For example, in accordance with the course of ageing of the oxidation catalytic converter that is preferably available for an entire operating period with a distance of approximately 200,000 km, a greater decrease in the low-pressure proportion of the recirculated exhaust gas is provided during the first 50,000 km than in the remaining 150,000 km. Depending on the operating point, the low-pressure proportion of the recirculated exhaust gas can decrease by about 100% in the case of a new, non-aged exhaust gas purification unit and up to 0% in the case of a marginally aged exhaust gas purification unit at the end of its operating period. The total exhaust gas recirculation rate, i.e. the proportion of the recirculated exhaust gas of the overall combustion gas that is fed back to the internal combustion engine in order to burn fuel, can vary to a large extent and typically ranges from 5% to 50%.
In an embodiment of the method, in the case of operating points of the internal combustion engine that are at least almost the same, the total exhaust gas recirculation rate is mainly kept constant with an increased operating period of the exhaust gas purification unit. In particular, it is provided that, with the same operating points in the load-speed engine characteristics of the internal combustion engine, the total exhaust gas recirculation rate will vary by less than 10% relatively over an operating period corresponding to a distance of approximately 150,000 km to 200,000 km. As a result of a total exhaust gas recirculation rate (for the same operating points) that is at least almost constant over the overall operating period of the exhaust gas purification unit provided, the untreated emission of contaminants from the internal combustion engine likewise remains almost constant over time with the same operating points, something which is advantageous for the designing of the exhaust gas purification unit. In accordance with the invention, in the course of the operating period, ageing-related efficiency changes in the exhaust gas purification unit or a related exhaust air purification component are mainly or almost exclusively compensated for by a reduction in the ratio of the low-pressure proportion to the high-pressure proportion with respect to the recirculated quantity of exhaust air.
In another embodiment of the method, an essentially decreasing low-pressure proportion is set with increasing mass flow of exhaust gas upstream of the divergence for the low-pressure proportion. As a result of this measure, the increase in the mass flow of exhaust air is counteracted, so that it is lower. Accordingly, the exhaust air purification unit will be increasingly less charged with an increased operating period and/or increased ageing with reference to the exhaust gas throughput. This enables at least a partial compensation of an ageing-related decrease in performance.
In another embodiment of the method, an essentially decreasing total exhaust gas recirculation rate is set with increasing mass flow of exhaust gas upstream of the divergence for the low-pressure proportion. At the same time, a further embodiment of the method provides for a greater decrease to be set in the low-pressure proportion than for the decrease in the total exhaust gas recirculation rate. As a result of these measures, the efficacy-reducing effect of an increasing mass flow of exhaust gas is advantageously counteracted.
In a further embodiment of the method, in an exhaust gas purification unit that is at least almost new, the low-pressure proportion is set to be at least twice as large as that of an exhaust purification unit operating period which corresponds to a distance for the associated motor vehicle of at least 100,000 km. As it has been possible to verify through trials, through these considerations, it is possible to compensate ageing-related decreases in efficiency in the exhaust gas purification unit and/or its exhaust gas purification components to an almost optimum extent.
In a further embodiment of the method, by means of an SCR catalyst, a reduction of the nitrogen oxides contained in the exhaust gas takes place downstream of an oxidation catalytic converter of the exhaust gas purification unit. An SCR catalyst is the term that is usually used to describe a catalytic exhaust gas purification component which is capable of catalysing a reduction of nitrogen oxides using ammonia as a reducing agent when there is an excess of oxygen. A selective catalytic reduction of nitrogen oxide can be provided either upstream and/or downstream of the divergence for the low-pressure proportion of the recirculated exhaust gas. It is particularly advantageous to provide a monolithic SCR catalyst, at least downstream of the divergence for the low-pressure proportion. In addition, an exhaust gas purification component that enables the selective catalytic reduction of nitrogen oxide can be provided upstream of the divergence for the low-pressure proportion. Preferably, also upstream of the divergence for the low-pressure proportion, a facility is provided for reducing particles in the exhaust gas, especially by means of a particle filter. As such, in this case, the exhaust gas purification unit upstream of the divergence for the low pressure proportion of the recirculated exhaust gas includes at least one oxidation catalytic converter and one particle filter. The aforementioned unit is preferably formed as a wall-perfused monolithic particle filter, especially one based on silicon carbide. Preferably, a catalytic coating is provided for the particle filter. This may, for example, exhibit an oxidation catalytic function or a function that promotes the oxidation of carbon particulate. Through this segmentation of the exhaust gas purification means, a particularly long-term stable exhaust gas purification is brought about in connection with the returning of low-pressure exhaust gas (as provided), especially with respect to nitrogen oxide reduction.
In a further embodiment of the method, the low-pressure proportion and/or a quantity of post-injected fuel can be set in such a way that the nitrogen dioxide proportion of nitrogen oxide contained in the exhaust gas (on the output of the oxidation catalytic converter and/or on the input of the SCR catalyst) is less than 70% (especially approximately 50% or less). If it is not possible to limit the NO2 content to these values by varying the low-pressure proportion, the method preferably provides for a late injection of fuel to take place so that these values are reached. This way, an optimization of the catalytic action of the SCR catalyst can be achieved. As a result of the decreasing low-pressure proportion of the recirculated exhaust gas over time, the optimization of long-term stability is made possible with comparable operating points. If necessary, the subsequent injection of fuel into one or more combustion chambers of the internal combustion engine test is preferably carried out at such a late point in the combustion cycle that the exhaust gas is enriched with non-combusted or partially combusted fuel components.
A motor vehicle internal combustion engine for carrying out one of the methods described above is advantageous, which has an air supply system for supplying combustion air to the internal combustion engine and an exhaust tract for admitting exhaust gas from the internal combustion engine, in which an oxidation catalytic converter, a particle filter and an SCR catalyst are arranged behind each other in the direction of flow of the exhaust gas. As such, an adding device for adding ammonia or a reduction means capable of separating ammonia is provided upstream of the SCR catalyst for the internal combustion engine in accordance with the invention. Furthermore, a first exhaust gas turbocharger is provided, the turbine of which is arranged upstream of the oxidation catalytic converter in the exhaust tract. A first exhaust gas recirculation line diverging from exhaust tract for returning exhaust air from the exhaust tract into the air supply system is provided upstream of the especially first exhaust gas turbocharger as well as a second exhaust gas recirculation line diverging from the exhaust tract between the particle filter and the SCR catalyst. At the same time, further adjusting means are provided for adjusting the quantity of exhaust air that is recirculated via the first and/or second exhaust gas recirculation line into the air supply system. The first exhaust gas recirculation line, which diverges upstream from the exhaust gas turbocharger turbine, represents a return path for high-pressure exhaust gas, via which a high-pressure proportion of the recirculated exhaust gas can be fed from the exhaust gas tract into the air supply system. On the other hand, the second exhaust gas recirculation line that diverges from the exhaust gas line between the particle filter and the SCR catalyst represents a return path for the low-pressure exhaust gas, via which a low-pressure proportion of the recirculated exhaust gas can be fed into the air supply system. As a result of the adjusting means provided in accordance with the invention, it is likewise possible to variably adjust the ratio of the low pressure proportion to the high pressure proportion of the total recirculated exhaust gas, as well as variably adjust the total quantity of recirculated exhaust gas.
In particular, the adjustment means for adjusting the low-pressure proportion or the high-pressure proportion of the total quantity of recirculated exhaust gas are also used, depending on the operating point of the internal combustion engine and the state of obsolescence (especially that of the oxidation catalytic converter), to adjust a nitrogen dioxide (NO2) proportion that is advantageous for the catalytic reduction of nitrogen oxide of the SCR exhaust gas purification component of the nitrogen oxides (NOx) that are present in the exhaust gas on the inlet side of the SCR exhaust gas purification component. Thus, an adjustment of the low-pressure proportion of the recirculated exhaust gas is particularly carried out in such a way that a proportion results (composed of NO2) which is lower than 70%. It is particularly preferable to adjust the low-pressure proportion in such a way that the NO2 proportion of NO2 contained in the exhaust gas is approximately 50%.
The addition of ammonia that is provided upstream of the SCR catalyst or the addition of a reducing agent that is capable of separating ammonia can be provided between the oxidation catalytic converter and the particle filter or additionally/alternatively, between the particle filter and the SCR catalyst, upstream or downstream of the point of divergence of the second exhaust gas recirculation line. In the case of a particle filter that is provided with a coating that is active for oxidation catalysis, it should be preferably arranged between the particle filter and the SCR catalyst. In the case of a coating of the particle filter with an SCR catalyst material, it should be preferably arranged between the oxidation catalytic converter and the particle filter.
Provision may be made for the means to comprise, for adjusting the recirculated quantity of exhaust gas, an adjustable restrictor element, arranged in the exhaust tract between the point of divergence of the second exhaust gas return line and the SCR catalyst and/or in the second exhaust gas return line before the point where it flows into the air supply system, and/or an adjustable restrictor element, arranged in the first exhaust gas return line before the point where it flows into the air supply system.
The particle filter can include a coating with an SCR catalyst material. This embodiment enables a selective catalytic reduction of nitrogen oxides using ammonia upstream of the divergence of the second exhaust gas recirculation line as well as downstream thereof. Even when the internal combustion engine is charged at a low level, the SCR catalyst material coating of the particle filter exhibits a temperature that is sufficient to reduce nitrogen oxide. Even under unfavorable conditions (where the SCR catalyst is below its start-up temperature downstream of the point of divergence of the second exhaust gas recirculation line) this allows for a sufficient reduction in nitrogen oxides. Furthermore, by increasing the quantity of exhaust gas recirculated via the low pressure path, conditions are enabled for the SCR catalyst to be relieved with respect to the quantity of exhaust gas flowing through it and in this way, allow for its efficiency to be improved.
A second SCR catalyst can be arranged in the second exhaust gas recirculation line. As a result, this enables a further increase in nitrogen oxide conversion. The second SCR catalyst in the second exhaust gas recirculation line allows for further relief, especially of the SCR catalyst arranged downstream of the divergence of the second exhaust gas recirculation line and, if necessary, also the SCR catalyst material coating of the upstream particle filter.
A second exhaust gas turbocharger can be provided, the turbine of which is arranged downstream of the turbine of the first exhaust gas turbocharger in the exhaust gas tract. This enables two-tiered charging of the internal combustion engine with a corresponding increase in efficiency with a comparatively reduced pollutant discharge.
A bypassable charge air cooler can be arranged in the air supply system for cooling compressed combustion air. This enables a variable decrease in the internal combustion temperature of combusted fuel in the combustion chambers of the internal combustion engine and thus a further decrease in the pollutant emission of the internal combustion engine, in particular with respect to nitrogen oxide.
An exhaust gas cooler can be arranged in the first exhaust gas return line and/or the second exhaust gas recirculation line in order to cool the exhaust gas that is recirculated to the air supply system. This measure also enables a decrease in the combustion temperatures. At the same time, it is advantageous if a bypass line, especially one with a flow regulator, is provided in a further embodiment of the invention for the exhaust gas cooler arranged in the first exhaust gas recirculation line and/or in the second exhaust gas recirculation line.
Further advantages, features and details of the invention arise from the description of preferred exemplary embodiments below, as well as with the aid of the figures. The features and feature combinations cited in the description above and the features and feature combinations cited below in the description of the figures and/or shown in the figures alone are not only usable in the respective combination as stated, but also in other combinations or individually, without exceeding the scope of the invention.